The present invention is generally directed to a seal assembly and is more particularly directed to a seal assembly with two separate sealing mechanisms that cooperate with one another to provide static and dynamic sealing.
A seal assembly for providing sealing between a movable cylindrical body and a housing body comprising a bore requires both static and dynamic sealing mechanisms. The seal assembly is normally associated with either the movable body or the housing body. That is, the seal assembly is normally in a fixed relationship with one of the bodies and provides a static seal with that body. A dynamic seal is normally provided against the other body, which is movable relative to the seal assembly.
Such conventional seal assemblies are employed for example, between reciprocating or rotating shafts disposed within a bore provided in a housing, or between a reciprocating piston and a cylinder. Conventional seal assemblies are disposed in an annular space between the movable body and the housing body and typically employ the same sealing mechanism for providing both static and dynamic sealing.
U-shaped spring-energized shaft seals are well known as disclosed, for example, in U.S. Pat. Nos. 5,163,692, 5,799,953, 5,979,904, 5,984,316 and 5,992,856.
The ""692 Patent, the ""953 Patent, and the ""904 Patent all disclose U-shaped spring-energized seals that rely upon a spring disposed between two flanges to bias the flanges from one another to enable both static and dynamic sealing. The ""316 Patent discloses a seal assembly with a spring that energizes a dynamic seal and a static seal that relies upon an interference fit of a flange between the housing and a metal band. The ""856 Patent discloses a seal assembly with independent static and dynamic sealing mechanisms, but both sealing mechanisms are spring-energized.
A particularly difficult application for such seal assemblies is cryogenic applications. A seal assembly employed in a cryogenic apparatus is subjected to an operating environment that is distinct from the operating environment of a non-cryogenic apparatus. A cryogenic operating environment is unique in many ways, including, for example, the effects of thermal contraction, the distinctive physical properties of cryogenic liquids, such as their high compressibility and volatility, and the effect of low temperatures on material properties and sealing capabilities. xe2x80x9cLowxe2x80x9d temperatures in the context of cryogenic applications are defined herein as temperatures below 190 degrees Kelvin where fluids such as nitrogen, oxygen, argon, methane, hydrogen and natural gas are in the liquid state.
At such low temperatures, fluoropolymers such as, for example polytetrafluoroethylene or polychlorotrifluoroethylene, which are typical materials used for the U-shaped body of conventional seal assemblies, shrink more than the typical metallic materials employed for housings and shafts, thereby often resulting in a tight seal against the shaft or piston and leakage problems between the seal and the housing bore or cylinder.
In cryogenic apparatus such as pumps, the seal assembly may also be subjected to very high differential pressures. For example, for a seal between the housing and the shaft of a cryogenic reciprocating pump, the differential pressure acting on the seal assembly may be higher than 5000 psi (34 MPa).
U.S. Pat. No. 5,996,472 discloses a cryogenic reciprocating pump that employs a U-shaped spring-energized seal assembly for sealing between a piston and the piston cylinder. The seal assembly is employed in combination with a plurality of separate piston rings that have underlying expander rings to press the piston rings against the interior surface of the cylinder. The U-shaped spring-energized seal, like other conventional spring-energized seals, relies upon a spring disposed between two flanges to press the seal against the interior surface of the housing. As temperature decreases and the effects of differential thermal expansion coefficients cause the seal material to shrink more than the housing, and the effectiveness of the seal is reduced.
The term xe2x80x9cthermal expansion coefficientxe2x80x9d is defined herein as the ratio of the change of size of an object to its original size per unit temperature rise. In the context of annular seal assemblies, changes in the xe2x80x9csizexe2x80x9d of an annular seal member result in a change in the inner and outer diameter. That is, as the temperature of an annular object is decreased, the inner and outer diameters will also decrease. Such dimensional changes will be greater for objects with higher thermal expansion coefficients.
Accordingly, there is a need for a seal assembly suitable for providing sealing between a movable body such as a shaft or piston and a relatively stationary housing or cylinder. In addition, there is a need for a seal assembly that is suitable for providing sealing in a cryogenic apparatus that compensates for thermal effects at low temperatures and thus reduces the potential for fluid leakage at both the static and dynamic sealing surfaces.
A seal assembly for providing fluid sealing between a movable inner body and a housing body employs two separate sealing mechanisms for providing static and dynamic sealing. The seal assembly is fixedly associated with one of the bodies, and the seal assembly comprises:
(a) a static seal for providing a seal between the seal assembly and the associated body; the static seal comprises a metallic member in the shape of a continuous ring wherein the static seal is temperature-activated by the metallic member having a thermal expansion coefficient that is different from that of the associated body. The metallic ring is thereby urged towards the associated body to activate the static seal when the seal assembly is within a predetermined operating temperature range; and
(b) a dynamic seal for providing a seal between the seal assembly and the body not in a fixed position relative to the seal assembly; the dynamic seal comprises a dynamic seal member in the shape of a continuous ring with at least one flange that cooperates with the static seal to provide a seal between the static and dynamic seal.
In preferred embodiments, when the temperature of the seal assembly is within the predetermined operating temperature range the metallic member forms a static seal by being pressed into direct contact with the associated body.
In one embodiment the associated body is the housing body and the movable inner body is a cylindrical body, such as a reciprocating piston, a piston rod, or a rotating shaft, which is disposed within a hollow cylinder or bore formed within the housing. In this embodiment, the operating temperature range, for example, may be in a range less than 190 degrees Kelvin. Within this operating temperature range the housing shrinks more than the metallic member to cause a temperature-activated static seal. The metallic member may be an alloy that has a thermal coefficient of expansion of less than 1.5xc3x9710xe2x88x926/xc2x0F. and the housing body is made from a material such as stainless steel that has a thermal expansion coefficient of about 9.9xc3x9710xe2x88x926/xc2x0F. The alloy selected for the metallic member may a nickel-iron alloy. In a preferred embodiment the alloy comprises between 34 and 36 per cent nickel, a maximum of 0.12 per cent carbon, a maximum of 0.50 manganese, and a maximum of 0.50 silicon, with the remainder being iron.
In an alternative arrangement, the associated body may be the movable body. In this arrangement, when the operating temperature range is in a range less than 190 degrees Kelvin, within the operating range the metallic ring shrinks more than the movable body to cause a temperature-activated static seal between the seal assembly and the movable body.
In a preferred embodiment the dynamic seal member comprises at least one flange that cooperates with a flange of the metallic member. Preferably at least a portion of the static sealing force applied between the flanges of the dynamic seal member and the metallic member is thermally activated.
In one embodiment the metallic member has a cross-section that is L-shaped with two surfaces facing the associated body and one surface facing the non-associated body. In another embodiment the metallic member has a cross-section that is J-shaped.
The dynamic seal member preferably comprises a fluoropolymer or thermoplastic member. Such materials are suitable for providing a dynamic sealing surface, which has a low coefficient of friction. Examples of suitable thermoplastic or fluoropolymer materials are materials selected from the group consisting of polytetrafluoroethylene, polychlorotrifluoroethylene, and ultra high molecular weight polyethylene.
The metallic member preferably further comprises first and second static seal surfaces that both face in the direction of the associated body. The first static seal surface being closer to the sealing surface of the associated body than the second static seal surface. The dynamic seal member also preferably comprises first and second seal surfaces that both face in the direction of the movable body with the first seal surface being a dynamic seal surface and closer to the sealing surface of the movable body than the second seal surface. A radial static sealing force is applied through the first static seal surface and a radial dynamic sealing force is applied through the dynamic seal surface. A second static seal is provided through cooperation between the respective second seal surfaces of the metallic member and the dynamic seal member (that is, the fluoropolymer or thermoplastic member).
The second static seal surface of the metallic member may further comprises a plurality of ridges. The ridges are oriented substantially perpendicular to the direction of movement of the movable body. For example, when the movable body is a rotating shaft, the ridges would be oriented substantially parallel to the longitudinal axis of the shaft. When the movable body is a reciprocating piston or piston rod, the ridges would be substantially perpendicular to longitudinal axis of the cylinder.
The dynamic seal member preferably further comprises a first flange with a surface facing the non-associated body and a second flange with a surface facing the associated body. A metallic member is preferably disposed between the flanges and applies a radial static sealing force through the second flange. When the seal assembly is subjected to a differential pressure the flanges preferably extend from a side of the dynamic seal member that faces the higher pressure. In this arrangement, the fluid on the high pressure side helps to urge the flanges outward for better sealing against the adjacent bodies.
In one embodiment, the metallic member of the static seal is not in direct contact with the associated body. For example, the metallic member may apply a radial static sealing force through an adjacent flange of the dynamic seal member while a spring member applies a mechanical sealing force to an opposite dynamic seal member flange that contacts the non-associated body.
The seal assembly of claim 15 wherein the dynamic seal member applies a radial sealing force outside the predetermined operating temperature range by being sized to provide an interference fit between the dynamic seal and the non-associated body.
A method for providing a fluid seal between a movable inner body and a housing body, is disclosed wherein a seal assembly is associated with one of the bodies by being in a fixed relationship thereto, the method comprising:
(a) thermally activating a static seal between the seal assembly and the associated body by employing a static seal element that has a different thermal expansion coefficient than the associated body, whereby the static seal element is urged towards the associated body when the seal assembly is within a predetermined operating range; and
(b) mechanically activating a dynamic seal between a dynamic seal element of the seal assembly and the body not in a fixed position relative to the seal assembly; and
(c) thermally activating a static seal between the static seal element and the dynamic seal element.
In a preferred method the operating temperature range is in a range less than 190 degrees Kelvin and within the operating temperature range the associated body shrinks more than the static seal member.
At least a portion of the mechanical forces for activating the dynamic seal may be provided by a spring member. In the alternative, or in addition to the spring member, the dynamic seal member may further comprise a resilient member that is sized to provide an interference-type seal that provides at least a portion of the mechanical forces for activating the dynamic seal.